Electrical insulation in medium- and high-voltage equipment is usually assured by means of using a dielectric gas which is introduced in a leak-tight complete enclosure in which live components of the electric equipment are arranged.
The most widely used dielectric gas in recent years has been SF6 because of its excellent dielectric properties and, among many other additional advantages, because it is not toxic for humans. However, this gas has an enormous environmental impact due to its high global warming potential (GWP=22800).
For this reason, alternatives that can replace this gas in such equipment have been sought in recent years. Different gases with good dielectric properties have been considered as an alternative to SF6 but for one reason or another (unacceptable toxicity, high global warming potential, etc.), they ultimately have not been implemented.
Likewise, the use of only more environmentally friendly gases such as dry air, N2, O2 or CO2 as dielectric medium in this equipment would entail a considerable increase in the size of such equipment for a given voltage level due to the lower dielectric strength of these gases with respect to SF6. Another option in this case would be to increase the equipment filling pressure to values exceeding those used with SF6 (around 1300 mbar), but this would involve conditioning design to fulfilling the various national regulations in force for vessels with pressures exceeding 1500 mbar, with the subsequent equipment cost increase.
A promising alternative are fluoroketones not only because they have good dielectric strength but also because some of them are not toxic for humans and have an environmental impact that is far inferior to that of SF6 gas. In fact, patent documents WO2010/146022 or WO2010142346 describes the use of fluoroketones for electrical insulation in medium- and high-voltage apparatus. Other patent documents, such as WO2012160158 and WO2012160155, describe mixtures of fluoroketones with vector gases such as CO2, N2, O2, air or mixtures thereof.
Another promising alternative are fluorinated nitriles, or fluoronitriles, which also have good dielectric strength. Patent document WO2013/151741 describes the use of two fluoronitriles as a dielectric fluid in electrical equipment.
Another problem negatively affecting the dielectric capacity of gaseous insulation systems is the presence of water molecules from materials used to manufacture some electrical components of the switchgear itself. Water can appear in the leak-tight complete enclosure of the switchgear in which the insulating gas is located because some thermoplastic materials used in manufacturing electrical components, such as polyamides, for example, may contain water therein. For example, in the case of polyamides, they can have between 4.5 and 7.5% by weight of water absorbed therein.
The presence of water in the gaseous medium lessens the dielectric properties thereof, so the presence thereof must be avoided.
Drying agents as well as molecular sieves have been used to solve the problem of water present in electric switchgear. A molecular sieve is a material containing small, precisely- and uniformly-sized pores and it is used as an adsorbent agent for gases and liquids. Molecules that are small enough to go through the pores are adsorbed, whereas larger molecules are not. Unlike a filter, the process works on a molecular level. For example, a water molecule may be small enough to go through same, whereas other larger molecules cannot.
In insulation systems with SF6 gas as the only insulating gas, separation of water is relatively simple by means of molecular sieves because the size of the SF6 molecule is substantially larger than the size of a water molecule, and therefore selection by molecule sizes does not pose a significant problem. However, this problem is not so easy to solve in gaseous insulation systems in which there are molecules with sizes comparable to the size of a water molecule, such as, for example, vector gases such as N2, CO2, dry air together with insulating agents as fluoroketones or fluoronitriles. In such dielectric insulating agents, the gases acting as vectors (CO2, N2, air, O2, etc.) have molecular sizes similar to that of a water molecule and the sieves can adsorb part of these gases instead of the water molecules.
Therefore, there is a need to develop gaseous dielectric insulation systems for medium- and high-voltage switchgear having a suitable dielectric strength (similar to that of SF6), which are not toxic for humans, which have minimal environmental impact and an insulation capacity that is not affected by the possible water that may appear inside the complete enclosures of said electrical switchgear, in which electrical components insulated with dielectric gases are arranged.